Many will be surprised to learn that a well designed and managed gravity-fed surface irrigation system has the potential to deliver on-farm application efficiencies in excess of 85% and up to 95% on the right soils. These efficiency levels are as good as pressurised sprinkler and drip systems. This form of surface irrigation is known variously as high-performance surface irrigation, precision surface irrigation or high-flow flood irrigation, and is becoming an increasingly popular alternative to pressurised systems in these times of rising energy costs. So what is required for high-performance surface irrigation? Firstly, let’s define what we mean by application efficiency.
On-farm application efficiency defined
On-farm application efficiency is generally defined as the amount of water consumed by the crop relative to the amount of water applied to the field (see the Food and Agriculture Organization of the United Nations for a detailed definition of water use efficiency). The application efficiency of surface irrigation is commonly reduced through runoff (or tailwater) at the end of the plot or by water infiltrating into the soil below the plant’s roots as illustrated in Figure 1 below.
Figure 1: Surface water runoff and infiltration below the root zone result in poor application efficiency
Improving application efficiency
By applying water at high flow rates (see here for detail on what constitutes high flow) you can reduce infiltration beyond the plant’s root zone (where it is not accessible), and by stopping your irrigation when the water reaches the end of your plot or furrow you can reduce or eliminate water lost through surface runoff.
So to achieve high efficiency, firstly you need to receive your water from your supplier at high flow rates. Secondly your on-farm application system needs to be capable of applying water at high flow rates, which means large bay gates or valves. Thirdly your on-farm system should be automated so that gates and valves can be programmed to open and close at a pre-determined time to optimise application efficiency (more on determining that time shortly). Additionally, for the best possible efficiency, flow measurement and in-field sensors are needed to manage application and to measure the advance of the water front and the amount of soil infiltration as the irrigation progresses.
The importance of irrigation duration
With high flow rates, stopping an irrigation even a little late will mean that large volumes of water are lost to surface runoff in a matter of minutes, as well as causing excessive waterlogging, quickly eliminating any efficiency gains up to that point and reducing crop growth. So irrigation duration (or run-time) becomes critical to reaching higher application efficiencies: with high flow rates, the optimal run-time is much shorter and the accuracy and precision required to manage it increases (see Figure 2 below).
Figure 2: With high-flows, irrigation cut-off becomes critical to achieving high efficiency
The logistics of manually opening and closing gates when irrigation durations are short can become very difficult to manage and most farmers complain of having to constantly ‘chase water’ as they open and close gates or valves as irrigation sequence progresses along a series of bays. So really you need to automate high-flow surface irrigation to ensure gates or valves close precisely at the right cut-off points and an alert message is sent if they fail to close.
Choosing the right cut-off point
The importance of managing irrigation duration (or run-time) is compounded by the fact that the optimal cut-off point in a given bay changes with each irrigation event because conditions such as crop density and soil moisture deficit change. Traditionally, cut-off points have been determined by rules of thumb based on years of experience or by physically visiting a bay to see the progress of the water and then making an assessment. More recently, decision-making is being aided by simple field sensors located halfway along the length of a bay to remotely indicate the progress of the water front.
Rubicon Water is currently working with researchers from the University of Southern Queensland to demonstrate and evaluate field sensors and software that dynamically calculates the optimal cut-off point in both bay (border-check) and furrow irrigation layouts. Sensors calculate flow rate and soil infiltration and software dynamically determines the optimal cut-off point for the current conditions, automatically closes the gate or valve and then schedules the opening of the next gate or valve in the sequence of bays. Not only does the system eliminate the guesswork required to determine irrigation duration, it also ensures cut-off points are automatically executed.
This technology is expected to achieve application efficiencies similar to aspersion and drip irrigation and to be repeatable in changing real-world conditions, making high-performance surface irrigation a lower cost on-farm modernisation alternative.